1. Field of the Invention
The present invention relates to a visual acuity examination apparatus where the contrast of an examination mark which is viewed by a person to be examined can be set and changed accurately.
2. Description of the Related Art
In a visual acuity examination or eyesight test, there is a case where a visual acuity examination is performed by changing the contrast of a mark for examination which is displayed on a eye examination table or in an eye examination apparatus. Such a visual acuity examination is performed so as to make such a diagnosis about whether or not an eye to be examined has caught cataract. In a conventional art, however, since a displayed mark whose contrast has been changed is viewed by a person to be examined, a display mark whose display concentration or density has been controlled by a printing technique or a display mark whose contrast is changed utilizing an optical filter or a polarizing plate is used.
However, in the conventional control system for controlling the contrast of a displayed mark, a complicated structure must be employed for realizing a quantified exact contrast of a mark and it is almost impossible to maintain the set contrast without deterioration with age.
In view of the above conventional art, an object of the present invention is to provide a visual acuity examination apparatus where an exact display contrast of a mark can be realized or attained with a much simplified structure and it is hardly deteriorated with age.
According to the present invention, there is provided a visual acuity examination apparatus comprising: a mark display face which has an outer face on which at least one mark is displayed so as to be viewed and which is formed such that the at least one mark and the periphery thereof are illuminated with a predetermined brightness; and a light shielding face which is disposed in front of the mark display face so as to be moved at a high speed, which is formed with a slit having a proper width and whose front face is formed so as to be illuminated with the same brightness as that of the mark display face.
In the above structure, the present invention can be structured such that the mark display face may be formed in a polygonal shape in order to change a mark of the marks displayed on the mark display face, which should be viewed by a person to be examined, or the mark display face is formed on an endless belt entrained about left and right guide rolls and a plurality of line-shaped marks are formed on an outer surface of the belt so that the mark of each line may selectively be positioned behind the slit. Also, the high speed movement of the light shield face having the slit can be realized by rotating a member forming a light shield face, or it can be realized by traveling this light shield face member at a high speed. Furthermore, in the apparatus of the present invention, a member where at least two slits having different widths are formed on one light shield face or a member where a slit is formed such that its width can be changed may be used. Alternatively, a plurality of light shield faces which are respectively formed with different slit widths are prepared and these faces can be used in an exchanging manner.
In the present invention, regarding illumination on the light shield face and the display face, when both the faces are formed with opaque material, light source light is irradiated on the faces from their front faces, and when both the faces are formed with semi-transparent and light diffusing material, light source light is irradiated on the both the faces from their back faces. Also, the remaining face of the light shield face where no slit is formed can be colored with, for example, white and black having the same area such that, when this light shield face is rotated or run at a high speed, the brightness of the face becomes approximately half the brightness of the mark display face.